Projection photolithography techniques are used for the production of photoresists intended for use in the fabrication of large scale integration components. For this purpose, a mask has to be projection printed to a photoresist-covered semiconductor wafer using an optical projection system. The heart of the optical projection system is a projection lens which reduces the pattern contained in the mask onto the resist layer on the semiconductor substrate. The high optical quality of the projection lens makes it possible to obtain the theoretical critical resolution for a narrow monochromatic spectral range.
A conventional transmission mask consists of a glass substrate covered with an opaque chromium layer with apertures to define the desired intensity pattern. The aperture pattern contains the spectrum information of the device geometries to be imaged on the wafer surface. The laws of diffraction cause the light intensity to spread into the dark regions surrounding the light beam. Constructive interference between the light fields diffracted by the apertures in the mask maximizes the intensity between the apertures, thus reducing the resolution of the projection system.
An improvement in resolution and contrast is highly valuable in advanced photolithography in order to meet the requirements of reduction in the lateral dimensions and minimum size of the pattern features of micro-electronic devices. An improved resolution can be achieved by using phase shift technique which needs so-called a phase-shifting mask. The phase-shifting mask differs from the conventional transmission mask in that a transparent phase-shifting layer covers adjacent apertures. The optical waves transmitted through adjacent apertures then follow different phases. If it is arranged that the emerging waves are 180.degree. out-of-phase with one another, destructive interference minimizes the light intensity between their images at the wafer surface, thus resulting in an improved resolution and contrast. However, phase-shifting masks are costly and difficult to manufacture because the phase structure must be closely related to the specific geometries of the chromium pattern. The geometry of the phase-shifting pattern must be calculated specifically and verified by actual experiment since heretofore no design rules or developing guidelines for mask design are available.
In addition, phase-shifting masks cannot be implemented satisfactorily for special geometries, e.g. for the end of large line groups, so that their applicability is limited.
Finally, the fabrication of defect free phase-shifting patterns is a key problem for industrial use of the phase-shift techniques. This problem calls for a suitable technology in respect of defect finding and repair.
Besides the resolution of imagery, the depth-of-focus (DOF) is a major concern in photolithography techniques. Several methods to improve the DOF has been proposed already. A particular approach consists in using a topographical mask which comprises a transparent layer having a refractive index greater than unity. The basic idea of this approach is increasing the optical path between the mask and the projection lens. The actual DOF only is increased with the use of such a topographical mask. No other improvement concerning resolution or exposure latitude is obtained however. In addition, the applicability of topographical masks is limited. For instance, such a mask cannot be applied to short frequency steps of the topology.
Another approach for improving the resolution and the depth-of-focus consists in performing off-axis illumination. By using a costly specially designed illumination system, the incident light beam is caused to be oblique to the mask. An improvement in DOF is obtained with periodic structures extending parallel to the x- or y-axis. With other structures which extend at an angle of 45.degree. relative to the coordinates system, off-axis illumination technique does not result in any improvement in resolution.